The process parameter can be, for example, the mass or volume flow rate of a medium flowing through a measuring tube. Of course, the parameter can also be fill level, pressure, temperature, moisture content, pH or the concentration of a substance in a medium, or some other process parameter.
In connection with the determining and/or monitoring of process parameters, the tendency is going increasingly in the direction of providing the user with, besides the actual measurement information, also information concerning the functioning or the operating life of the measuring device under the existing system- and/or process-conditions. Current catchwords in this connection are ‘Predictive Maintenance’ and ‘Mean Time Before Failure’. The goal of these efforts is, in the final analysis, to exclude, or at least to reduce to a minimum, measuring device downtime.
A measuring device includes, usually, a sensor and a transmitter, with the latter containing the electrical and/or electronic components for measurement signal conditioning, for obtaining the measured value and/or for data transmission to a remote control location. Differences may exist between so-called compact devices, where sensor and transmitter form an integral unit, and measuring devices, where the sensor and the transmitter form units separated from one another.
In the case of industrially-used measuring devices, climatically difficult conditions often prevail. These are caused e.g. by more or less severe humidity- and temperature-changes, or by cleaning activities at the measuring device or in the vicinity of the measuring device. In order to protect moisture-sensitive electronic components in the transmitter housing from condensate, all joints and openings, such as lids, threaded cable connections, seams, and other seals must be made gas-tight. Under the previously-mentioned environmental conditions, it is still not to be ruled out that over time the seals will begin to be compromised. With frequent temperature changes, differences in pressure can arise between the interior of the housing and the surrounding environment. These differences are usually smaller than 200 mbar. A lower pressure in the transmitter housing can result in moist, warm air from the environment gradually being drawn into the interior of the transmitter. Through a kind of pump-effect, the air in the transmitter housing can gradually absorb so much moisture that, with a subsequent drop in the surrounding temperature, a liquid condensate becomes present in the transmitter housing. In the extreme case, this pump-effect leads to a so-called “flooding” of the transmitter, i.e. the electronic components come into direct contact with the condensate.